Good Practices for Water Quality Management in Research Reactors and Spent Fuel Storage Facilities | IAEA

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Basic Principles

Objectives

IAEA Nuclear Energy Series

Technical Reports

Good Practices for Water Quality

Management in

Research Reactors and Spent Fuel Storage

Facilities

No. NP-T-5.2

Guides

IAEA Nuclear Energy Series No. NP-T-5.2Good Practices for Water Quality Management in Research Reactors and Spent Fuel Storage Facilities

INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA

ISBN 978–92–0–112810–2

ISSN 1995–7807

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GOOD PRACTICES FOR

WATER QUALITY MANAGEMENT IN RESEARCH REACTORS

AND SPENT FUEL STORAGE FACILITIES

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AFGHANISTAN ALBANIA ALGERIA ANGOLA ARGENTINA ARMENIA AUSTRALIA AUSTRIA AZERBAIJAN BAHRAIN BANGLADESH BELARUS BELGIUM BELIZE BENIN BOLIVIA

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GOOD PRACTICES FOR

WATER QUALITY MANAGEMENT IN RESEARCH REACTORS

AND SPENT FUEL STORAGE FACILITIES

IAEA NUCLEAR ENERGY SERIES No. NP-T-5.2

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IAEA Library Cataloguing in Publication Data

Good practices for water quality management in research reactors and spent fuel storage facilities. — Vienna : International Atomic Energy Agency, 2011.

p. ; 29 cm. — (IAEA nuclear energy series, ISSN 1995–7807;

no. NP-T-5.2) STI/PUB/1492

ISBN 978–92–0–112810–2 Includes bibliographical references.

1. Research reactors — Water quality management. 2. Water cooled reactors — Corrosion. 3. Water — Purification. I. International Atomic

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July 2011 STI/PUB/1492

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FOREWORD

Water is the most common fluid used to remove the heat produced in a research reactor (RR). It is also the most common media used to store spent fuel elements after being removed from the reactor core. Spent fuel is stored either in the at-reactor pool or in away-from-reactor wet facilities, where the fuel elements are maintained until submission to final disposal, or until the decay heat is low enough to allow migration to a dry storage facility.

Maintaining high quality water is the most important factor in preventing degradation of aluminium clad fuel elements, and other structural components in water cooled research reactors. Excellent water quality in spent fuel wet storage facilities is essential to achieve optimum storage performance. Experience shows the remarkable success of many research reactors where the water chemistry has been well controlled. In these cases, aluminium clad fuel elements and aluminium pool liners show few, if any, signs of either localized or general corrosion, even after more than 30 years of exposure to research reactor water. In contrast, when water quality was allowed to degrade, the fuel clad and the structural parts of the reactor have been seriously corroded.

The driving force to prepare this publication was the recognition that, even though a great deal of information on research reactor water quality is available in the open literature, no comprehensive report addressing the rationale of water quality management in research reactors has been published to date.

This report is designed to provide a comprehensive catalogue of good practices for the management of water quality in research reactors. It also presents a brief description of the corrosion process that affects the components of a research reactor. Further, the report provides a basic understanding of water chemistry and its influence on the corrosion process; specifies requirements and operational limits for water purification systems of RRs; describes good practices for water chemistry control in research reactors; defines parameters recommended, techniques applicable, sampling procedures and sampling frequency to monitor water quality in RRs, and describes the importance of a quality assurance programme, and the implementation of a corrosion surveillance programme (CSP) as part of the water management programme. Whenever applicable, considerations are made for primary cooling system, spent fuel storage basins, secondary cooling system, emergency cooling systems, make-up systems and water reservoirs of RRs.

The IAEA acknowledges the contributions of R. Haddad, Comisión Nacional de Energía Atómica, Argentina;

L.V. Ramanathan, Instituto de Pesquisas Energéticas e Nucleares, Brazil; F.P. Bakker, Energy Research Centre of the Netherlands; G.J. de Haas, Nuclear Research and Consultancy Group, Netherlands and R.L. Sindelar, Savannah River National Laboratory, USA, in the preparation of this report. The IAEA technical officers responsible for this report were P. Adelfang and A.J. Soares of the Division of Nuclear Fuel Cycle and Waste Technology.

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EDITORIAL NOTE

This report has been edited by the editorial staff of the IAEA to the extent considered necessary for the reader’s assistance. The views expressed are not necessarily those of the governments of the IAEA’s Member States.

Although great care has been taken to maintain the accuracy of information contained in this publication, neither the IAEA nor its Member States assume any responsibility for consequences which may arise from its use.

The use of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries.

The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA.

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1. INTRODUCTION

For over 60 years, research and test reactors (hereinafter referred to as ‘research reactors’¹) have made valuable contributions to the development of nuclear power, basic science, materials development, education, training, and radioisotope production for medicine and industry. In total, 660 research reactors have been built since the Chicago graphite pile CP-1 went critical in December 1942. From these, 239 are operational and 236 are in permanent shutdown, without having been decommissioned [1]. Considering that the majority of shutdown reactors still have nuclear fuel elements within the facility, they require, for safety reasons, a continuous maintenance programme, especially to avoid corrosion of fuel elements to maintain fuel integrity, prevent release of radioactivity, and ensure safe handling throughout the storage period.

Because research reactors have a variety of applications, such as validation of reactor physics codes, training, neutron activation analysis and radioisotope production, several types were built. Some reactors are pressurized;

others are open pool type. Some use heavy water but most use light water as the neutron moderator. The various reactors’ cores are loaded with fuels of very different designs, such as single fuel rods, fuel bundles, or plates assembled in fuel elements or sometimes termed ‘fuel assemblies’. The reactors also have a variety of fuel compositions, core configurations and power levels, ranging from ‘zero power’, also known as critical facilities, to hundreds of megawatts.

Regardless of the reactor type, its application, composition or power level, in the majority of them, water is used as the core cooling fluid, moderator and biological shielding. As the cooling fluid, water removes the heat produced by the fission reaction and transports it to the heat exchanger system; as moderator, the water slows down the high energy neutrons produced in the fission process, making them energetically favourable for new fission reactions required to sustain the reaction chain; and as radiological shielding, it attenuates the radiation emitted in the reactor core in order to assure a safe environment for the reactor operators. Being an efficient agent for all three purposes, it can produce undesirable conditions if its quality² is poor. Dispersed impurities may become activated by the neutron flux as the water circulates through the reactor core, and if their concentration is too high, it can result in unwanted high radiation levels. Also, low quality water can lead to crud accumulation, decreasing thermal conductivity, with a consequent fuel temperature increase, which accelerates oxidation of the cladding and causes further decrease of thermal conductivity, in a vicious loop that may result in fuel failure. On the other hand, if the water chemical purity is assured, no elements other than the water components (e.g. deuterium in heavy water) will be available for activation, heat transfer from the fuel to the coolant will be high, and electrical conductivity of the coolant will be low, resulting in an environment with a very low corrosion rate for both the fuel and structural materials in the reactor system.

Because water is the main substance in contact with the fuel elements, maintenance of very high quality water is the most important factor to prevent degradation of the fuel clad and structural components of the reactor. In a research reactor, water deterioration can occur for various reasons: use of a poor quality water source for water make-up, incorporation of environmental dirt, biological activity and primary circuit corrosion. Because water quality can deteriorate, maintenance of good water quality is a key feature of the reactor maintenance programme, and it is essential to achieve optimum fuel performance, either in the reactor core during reactor operation, or in the spent fuel storage pool, after the fuel reaches its maximum burnup level.

Fuel elements used in research reactors are made primarily of aluminium alloy claddings. Stainless steel and zirconium alloys (e.g. Zircaloy) are also used as cladding materials for the fuel, but they represent a very small percentage of the total fuel elements produced worldwide. The oxidation levels of these alloys in normal operating conditions (with fuel/water interface temperatures below 100ºC) in general pose no limiting operating condition for the reactor. However, stainless steel and Zircaloy have some disadvantages when compared to aluminium. Stainless steel constituents have higher absorption cross sections and production of activation products with higher half-lives compared to aluminium cladding alloys and Zircaloy. In particular, cobalt-60, produced from activation of cobalt as an impurity, and by capture reactions with nickel in austenitic stainless steels, causes a significant radiation dose

1 In this publication, the term ‘research reactor’ also includes critical and sub-critical assemblies.

2 Water quality is defined by a set of parameters used to characterize the water physical and chemical conditions. It includes pH, conductivity, dissolved impurity species, undissolved solids, colloids, organic substances, biological organisms and temperature.

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due to its high energy gamma decay. In addition, Zircaloy is more expensive than aluminium, and aluminium based fuel core materials with aluminium cladding are readily fabricated. The challenges and higher costs for fabrication, disassembling and decommissioning operations for stainless steel and Zircaloy compared with aluminium are the primary reason that most of the fuel used in research reactors, in both eastern and western countries, is fabricated with a core consisting of uranium–aluminium alloys or dispersion and protected by an aluminium alloy cladding.

There are important considerations, however, in the use of aluminium and aluminium alloy claddings for reactor fuel during reactor operation and in post-reactor fuel storage in water. Aluminium alloys show very different oxidation behaviour from fuels produced with stainless steel and Zircaloy claddings. During reactor operation in an aqueous environment, aluminium claddings can develop relatively thick oxide layers. Oxide layer thicknesses of up to 61 µm have been reported under high heat flux conditions [2]. Depending on the reactor operating condition, there is the risk of high corrosion, oxide spalling, blistering and intergranular attack of the aluminium fuel clad materials when the thicknesses of the oxide layer reaches about 25 µm [3, 4]. Spalling can also occur with oxide layers on the order of 10 µm [5]. Therefore, it is important to consider the conditions of reactor operation for aluminium clad fuels to avoid excessive oxide layer buildup that leads to a high metal/oxide interface temperature, especially when the reactor operates under high heat flux conditions.

Water is also the most common media used to store spent fuel elements³ from research reactors after being removed from the reactor core. Spent fuel is stored either in the reactor pool or in some away-from-reactor (AFR) wet facilities, where the fuel elements are safely maintained until they can be moved to a dry storage facility, or until they are retrieved for final disposal or reprocessing. There are some documented cases of aluminium clad spent fuels that have been in water storage for more than 40 years and remain in pristine condition, while others are severely degraded by pitting corrosion after a few years of exposition to low quality water, causing serious concerns, since pitting corrosion can eventually lead to breach of the cladding material and release of radioactivity to the storage basin [6]. Not surprisingly, in a survey conducted by the IAEA in 2002, addressing the concerns expressed by research reactor operators over their spent fuel management programmes, research reactor materials degradation was one of the main concerns, together with final disposal of the spent fuel and limited spent fuel storage capacity. Information gathered in this survey is part of the IAEA’s restricted access Research Reactor Spent Fuel Database (RRSFDB).

The IAEA recognizes that, although a great deal of information on reactor water quality is available in the open literature, only a few publications address the rationale of water quality management in research reactors. The IAEA also understands that such a document can help research reactor operators in implementing water quality management programmes in their facilities, and with this objective the IAEA supported the production of these guidelines to provide a comprehensive catalogue of good practices in water quality management for research reactors. The report was produced with the main purpose to give research reactor operators and managers insight into basic principles of aluminium corrosion, water chemistry and current good practices in the reactor ageing/safety management adopted in a number of Member States, thus helping them manage their own problems associated with reactor ageing, in particular, corrosion problems that can arise by an inadequate water quality maintenance programme.

The publication is mainly intended for operators and managers of facilities dealing with aluminium clad fuel and aluminium structural components in the reactor pool, and in the spent fuel storage pool. It emphasizes requirements for open pool reactors, mainly Material Test Reactors (MTRs) and TRIGA type reactors using ordinary water as the surrounding environment for the reactor core. The report does not include procedures or good practices for water quality management in reactors that use heavy water as the surrounding environment for the reactor core. However, this report can be used by operators and managers from other reactors, especially those in which stainless steel is used. In this case, it is important to recognize that stainless steel is more resistant to water corrosion than aluminium, and therefore, the requirements for stainless steel structures and fuel cladding may be relaxed.

It is also important to mention that an important part of this report is based on knowledge gained during a co-ordinated research project (CRP) on Corrosion of Research Reactor Aluminium Clad Spent Fuel in Water — Phases I and II, organized by the IAEA from 1996 to 2005; findings of the CRP have been published [7, 8].

3 Fuel is regarded as spent nuclear fuel, regardless of burnup level, when it is discharged from the reactor core for the final time, and put in a certain place, in the reactor pool, or in a pool away from the reactor, for removal of the residual heat.

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Finally, considering that practices for maintaining integrity of the fuel and structural components of research reactors are part of the overall strategy for safe operation of a research reactor, it is necessary to mention that the recommendations and practices described in this report comply with IAEA Safety Standards Series No. NS-G-4.2 (Maintenance, Periodic Testing and Inspection of Research Reactors), NS-G-4.3 (Core Management and Fuel Handling for Research Reactors) and NS-G-4.4 (Operational Limits and Conditions and Operational Procedures for Research Reactors).

2. WATER CHEMISTRY

2.1. IMPORTANCE OF WATER QUALITY IN RESEARCH REACTOR SYSTEMS

As any other chemical substance, water has a certain potential to interact with the materials of components within the water. The type and extent of interaction is, inter alia, determined by the physical state and composition of the water and its parameters such as temperature and pressure to which the materials under consideration are exposed. Careful selection of suitable materials for any functional, water bearing, component is of utmost importance, as interaction between the water and this component may affect the chemical and mechanical properties of the latter. Prolonged reaction may lead to degradation (corrosion) and loss of the structural integrity of the component. Selection is therefore restricted to materials known to show very limited interaction with water during prolonged exposure under a range of conditions. An example of such a material is aluminium. Under certain conditions, the properties of aluminium are hardly influenced by constant exposure to water. This and other favourable properties such as its density, mechanical properties, ease of fabrication and neutronic properties make aluminium a widely used construction material for research reactors.

The actual situation is complicated because water commonly contains a wide range of dissolved and non-dissolved species in various concentrations: metal ions, colloids, gasses and dust particles. Some of these species have a profound influence on the interaction between water and materials. Under certain conditions, the corrosion resistant character of a material like aluminium is even undermined.

In this section, a brief overview of the chemistry of water is presented with a focus on the prime parameters and processes that are relevant for the interaction between water and aluminium. The focus is on the processes that take place or could take place in the primary cooling system. However, it should be noted that these processes apply to all other water-bearing systems in a research reactor such as storage basins, decay tanks, the secondary cooling circuit and water make-up systems.

2.2. CHEMICAL COMPOSITION OF WATER

The chemical compound water is almost entirely made up of H₂O molecules, with the exceptions that approximately one atom in 6500 is deuterium, and that water in the liquid state is dissociated (ionized) into H⁺ and OH^– that is dependent on temperature. However, the same term ‘water’ is also commonly used for aqueous solutions such as tap water and impure cooling water. Such solutions consist of more than 99% water, the remainder consists in a wide range of compounds other than H₂O. Although present in only small amounts, some of these compounds may have a profound influence on the properties of the medium water present in a research reactor.

The nature and concentration of non-pure H₂O compounds is the product of reactions and interactions between the medium water and its environment. On circuits with forced circulation, the kinetic energy of the water stream causes erosion of pipes and components of the circuit. During erosion, very small fractions of these components are mechanically released in the form of particles that are transported downstream, to be deposited as sediments, or dissolved in the form of charged particles: cations (e.g. Na⁺, Ca²⁺, K⁺) and anions (e.g. Cl^–, CO₃^2–, SO₄^2–). The capacity of water for dissolved species in a solution is limited: if the amount of dissolved species exceeds the ‘solubility product’ of a certain compound, this compound will precipitate from the water in the form of salts.

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Additionally, there is a continuous interaction between the water and the atmosphere in contact with it.

Components from the atmosphere such as O₂ and CO₂ will — to a certain limit — dissolve in water. These dissolved gaseous species will also exert influence on the composition and the reactions that take place in water.

Another factor of great importance is the temperature. Most chemical reactions are temperature controlled as heat provides the energy necessary for the reactions to occur. An increase of temperature generally tends to accelerate reactions and increase the capacity of water for dissolved species.

The cooling system of a research reactor is not a very complex one. In this system, the number of variables involved is very restricted. This, and the restricted size of the cooling system, make the composition of the (cooling) water less prone to variations, allowing the establishment of very simple water management programmes, with countermeasures to keep the original composition, such as, for instance, water purification. Management of cooling water aims at keeping the water as ‘pure’ as possible, i.e. keeping the amount of non-H₂O species as low as technically possible. Yet, interactions between the water and the contacted material (aluminium) and the atmosphere cannot be avoided. Using the basic principles of water chemistry, it is possible to model these interactions and to quantify their contributions to the composition of the cooling water. In the next sections, some of these principles will be highlighted in the light of the aluminium-water system. The specific limits and recommended ranges for water quality parameters for each reactor water system are discussed in Section 6 and are reproduced in Appendix I.

2.3. pH

In the management of water cooled systems, the pH together with conductivity are two of the foremost parameters used in characterizing the quality of the water. The reason for this is that the interaction of water with metals is heavily controlled by the pH of the water. Accurate determination and registration of the pH is therefore of utmost importance. In this section, some of the background behind the pH parameter is highlighted.

Pure water is a poor electrical conductor, yet the observation that it weakly conducts electric current indicates that pure water contains ions (charged particles). It appears that part of the water is dissociated (ionized) into H⁺ and OH^–. In reaction form:

H₂O  H⁺ + OH^– (1)

The  symbol indicates that this reaction is an equilibrium reaction, i.e. the reaction is reversible. The tendency for the reverse reaction (to the left) to occur, however, is more likely than the ionization reaction (to the right), that is, only a very small part of the H₂O is ionized. At 25C, 10^–5% of the water molecules are ionized, i.e.

the concentration of H⁺ ions, and thus also of OH^– ions (denoted as [H⁺] and [OH^–]), in pure water amounts 10^–7 mol/L. As for any equilibrium reaction, an ‘equilibrium constant’ K is defined, and for this specific reaction K is:

K_w = [H⁺] * [OH^–] (2)

K is, therefore, the mathematical product of [H⁺] and [OH^–]. At T = 25C K_w is 10^–14, a constant for any aqueous solution at 25C. As with any equilibrium reaction, the dissociation reaction of water is temperature dependent. At 60C, K_w is 9.62*10^–14, corresponding to H⁺ and OH^– concentrations of 3.1*10^–7 mol/L; at 100C the concentrations of H⁺ and OH^– in pure water amount to 7.42*10^–7 mol/L. In general, an increase of the temperature will result in higher H⁺, and OH^–, concentrations.

Mathematically, the value of pH is defined as the negative ‘log’ of the H⁺ concentration when expressed in moles per litre:

pH = –log[H⁺] (3)

Therefore, an H⁺ concentration of 10^–7 mol/L corresponds to a pH value of 7. Similarly, a pH of 5 means that the H⁺ concentration is 10^–5mol/L, still very low, yet 100 times higher than in a solution with pH 7.

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A solution with a pH value of 7 is ‘neutral’; solutions with a pH value less than 7 are termed ‘acidic’; and those with pH between 7 and 14, ‘basic’. In acidic solutions, H⁺ is dominant over OH^–, whereas in basic solutions, the opposite is the case. However, for a given temperature, the equilibrium constant K_w, product of [H⁺] and [OH^–] remains the same. Therefore, assume an aqueous solution with an H⁺ concentration of 10^–1 mol/L at T = 25C. For this temperature, K_w is 10^–14, then the concentration of OH^– can be calculated as [OH^–] = 10^–13 mol/L.

Perfectly pure water has a pH value of 7. However, this is only the case for an entirely isolated volume of water. In most cases, water is exposed to an atmosphere with a certain composition like air. As explained in Section 2.2, components from the atmosphere will be taken up (dissolved) in the water and subsequently react with it. A prominent air component is CO₂, which dissolves and reacts with water according to the reactions:

CO₂ (g) = CO₂ (l) (the equilibrium between dissolved CO₂ in water and air)

CO₂ (l) + H₂O (l) = H₂CO₃ (l) (4)

H₂CO₃ + H₂O = H₃O⁺ + HCO₃^– (5)

HCO₃^– + H₂O = H₃O⁺ + CO₃^2– (6)

The ultimate effect of CO₂ uptake by the water from the atmosphere is formation of more H⁺, with consequent lowering of the pH.

The pH also depends on the partial CO₂ pressure: the higher the CO₂ pressure, the more CO₂ is dissolved and thus the lower the pH becomes. Pure water at 25°C in equilibrium with air with a CO₂ concentration of 338 ppm has a pH of 5.7, assuming that the air is in equilibrium with the water, i.e. that the CO₂ uptake has reached its maximum and is perfectly mixed with the water.

The case of CO₂ uptake is a good illustration of an interaction between water and its environment which has a significant impact on its composition. Most natural systems have a tendency to restore equilibrium after a disturbance. In more complex systems, lowering the pH may therefore trigger counter reactions, resulting at an increase of the pH in order to restore the former equilibrium. The ultimate pH is then the product of a series of reactions, the number of the reactions being a function of the number of species present. The capacity of a system to restore equilibrium is commonly referred to as the ‘buffering capacity’.

2.4. CONDUCTIVITY

Conductivity is a very important parameter related to corrosion of metals in an aqueous medium. By definition, conductivity is simply a measure of the ability of a medium to carry an electric current. As stated, conductivity is an index of how easy it is for electricity to flow in the medium. In water it is caused by the presence of some dissolved ionic species. Ions pass electric charge from one ion to the next. This means that the more anions and cations contained in water, the more electricity is carried, and the higher the conductivity. Therefore, pure water has a very low conductivity, and seawater has a very high conductivity. The conductivity of water is expressed in S/cm (S = siemens), or more usually, in μS/cm, where siemens is the inverse of the resistance; 1 S = 1/ohm. At room temperature, the conductivity of pure water at 25°C is 0.0548 S/cm. Seawater, on the other hand, has a conductivity of about 40 000 μS/cm.

It is understood that even in purified water it is impossible to avoid some degree of impurities. Important sources of impurities are corrosion of components of the cooling system including the dissolution of (metallic) impurities in the piping, dust that falls into the pool, the air in contact with the water in the pool surface, aerosols and detergents. Typical dominant impurities are Al³⁺, Fe²⁺, NO₂^–, HSO₃^–, NO₃^–, SO₄^2– and Cl^–. Each ion makes a contribution to the total conductivity of the aqueous solution. One parameter used to estimate the overall conductivity of an aqueous solution with ions present in it, is the molar conductivity, Λ, defined as the conductivity (in S/cm) divided by the molar concentration of the ion in the solution, expressed in mol/cm³. Table 1 [9] gives the molar conductivity for some ions important for water quality control in research reactors. By using this table, it is possible to estimate the contribution of these ions to the conductivity of an aqueous solution.

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Frequent element specific analyses of water samples are one option to monitor its quality. They provide detailed information about the concentrations of all elements present. However, this is time consuming and not a practical solution for an on-line measurement system. Instead, the overall conductivity of the water is measured.

Continuous or frequent measurement of the conductivity is a good method to detect sudden or gradual increase in the bulk concentration of ionic species. Element specific analyses are then required to determine the cause (origin) for the increase.

The conductivity of water is reduced by adding, to the original bulk, water with lower concentrations of non-H₂O ionic species, or circulating the water through a water purification system, in which these ionic species are removed from the water. In a research reactor, it is not recommended to add any chemical solution to the reactor cooling water, to avoid eventual activation with consequent increase in radiation dose. Therefore, the only recommended alternative is to circulate the water through the water purification system, keeping the purification system (see Section 4) as required to maintain water quality limits.

Considering that pH and conductivity are based on ion concentrations, as expected, there is a correspondence between the two values. Figure 1 shows the relation between conductivity and pH, considering the presence of hydrochloric acid in pure water. The values for other acids will be comparable.

Although the pH value of water is an indication of the quality of water, a low conductivity is a more reliable indication for low corrosion potential. A solution of 0.01 molar KCl will have a pH value comparable to ultrapure water, but will have a conductivity of about 15 mS cm^–1.

With a pH value between 4.5 and 7 and conductivity below 1 μS.cm^–1 corrosion of most metals is minimal.

2.5. ACTIVATION PRODUCTS IN REACTOR PRIMARY COOLANT WATER

There are numerous water activation products and activated water impurities in the reactor primary water. The species that are significant include: ³H, ¹³N, ¹⁶N, ¹⁸F, ²⁴Na and ³⁸Cl.

2.6. FORMATION OF IONS IN WATER, OXIDATION-REDUCTION

Oxidation-reduction reactions are among the most prominent type of reactions in chemistry. The basic principle behind these reactions is that most elements have a tendency to donate part of their electrons to elements with a larger affinity to accept electrons. The potential (tendency) of a metal to donate electrons largely depends on the distribution of the electrons in the metal atom. Magnesium and aluminium are examples of metals with a great tendency to donate electrons in electrochemical reactions. In contrast, nickel and titanium are more resistant to

TABLE 1. MOLAR CONDUCTIVITY^aFOR VARIOUS COMMON IONS [9]

Ion Λ [(S/cm)/(mol/cm³)]

Cl^– 76.3

NO₂^– 71.8

NO₃^– 71.4

HSO₃^– 58

SO₄^2– 80

HCO₃^– 44.5

OH^– 198

H⁺ 349.6

K⁺ 73.5

Na⁺ 50.1

a Considering infinite dilution and temperature equal to 25^oC.

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oxidation; their tendency to act as an electron donor is much less. Platinum and gold are noble metals; they are the least susceptible to oxidation.

Most metals do not occur as metallic species in nature. For example, Fe and Al are found as oxides (e.g.

Al₂O₃), sulphides (e.g. FeS₂) and carbonates (e.g. FeCO₃), etc. Formation of these compounds involves oxidation- reduction reactions, with the metals donating part of their electrons. For instance, Al₂O₃ may be written as Al₂³⁺O₃^2–. The process of metal oxidation is also known as corrosion, the result of a chemical reaction, with consequent deterioration of the properties of the metal.

A well known example of metal oxidation is the corrosion of iron (rust formation) by water in the presence of oxygen.

Fe  Fe²⁺ + 2e^– (7)

+

H₂O + 1/2O₂ + 2e^– 2OH^– (8)

___________________________ 

Fe + H₂O + 1/2O₂  Fe(OH)₂ (9) In this case, iron donates electrons to oxygen, resulting in the formation of iron oxide. The reaction (9) is a summation of two half-reactions: an oxidation (7) and a reduction (8) reaction. The oxidation and reduction reactions are also referred to as the anodic and cathodic reactions, respectively. Reactions (7)–(9) are an example of an electrochemical reaction. Note that the presence of water is vital. This is the reason that iron hardly corrodes when exposed to air with low humidity.

In aqueous environments, oxidation-reduction reactions are a common feature. Dissolution of metals occurs by means of oxidation-reduction reactions. For aluminium in water, the aluminium will donate three electrons:

Al  Al³⁺ + 3e^– (10)

In this case, the acceptor is H⁺:

2H⁺ + 2e^–  H₂ (g) (11)

Therefore, oxidation of aluminium promotes the formation of (small) amounts of hydrogen at the expense of H⁺. This example illustrates the close connection between pH and oxidation-reaction. In an acidic environment (pH < 7), there are relatively many H⁺ ions that combine to H₂ by accepting electrons. More H⁺ ions means that more electrons are required, and they have to be donated by the Al. This increasing need can only be met with more dissolution of Al; i.e. the lower the pH, the more aluminium will be dissolved.

Thermodynamic models provide insight to the reactions that are likely to occur in a system. However, the results do not provide a clue on the speed with which a reaction will proceed. The results are time independent. At

FIG. 1. Relation between conductivity and pH for an aqueous solution with HCl.

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any moment in time, the reaction will occur as each reaction has its own ‘kinetics’. Also, the physical form of the reaction products may be decisive for the progress of the reaction. In case of the corrosion of aluminium, Al(OH)₃ is formed. In nature, several varieties of Al(OH)₃ are known to occur dependant on the conditions that aluminium is exposed to. In case of reactor fuel at high temperature, the oxidation product has a very dense structure, which makes the underlying aluminium inaccessible for the cooling water, i.e. the oxidation process comes to an end.

Consequently, the oxidation product protects the bulk of the aluminium from further corrosion. This process is also referred to as passivation.

The rate of metal corrosion in a water environment can be strongly affected if the metal is in contact with another metal, resulting in a special type of electrochemical reaction known as ‘galvanic corrosion’. Galvanic corrosion is a process that occurs in a water environment, involving two coupled metals with different potentials.

Assuming the difference between their potentials is large enough, the metal with the highest potential will donate electrons, whereas the other metal will act as an acceptor; i.e. one metal may cause corrosion of the other. The role of water in electrochemical reactions is essential. It acts as the medium by which the aqueous species involved in the reactions are transported. In addition, it plays a key role in the flow of the electric current (electrons).

Table 2 [10] shows a typical galvanic series for some metals and alloys in seawater. It is based on corrosion potential measurements in seawater, and shows the relative potential/nobility of the metal/alloy. According to this table, metal/alloys of a lower position in the table have their corrosion rate increased when placed in seawater and in contact with metal/alloys of an upper position. The relative position can be different for other environments.

Metallic species more noble (i.e. higher position in the table) than a reactor material would tend to cause oxidation of the material and its dissolution. For example, copper ions in water are detrimental to aluminium. Mercury is forbidden in aluminium systems for this same reason.

2.7. EFFECT OF DISSOLVED ANIONS IN ALUMINIUM STRUCTURES

For systems with components and structures made of aluminium, dissolved anions such as Cl^–, HCO₃^– and SO₄^2– are of special interest because of their capacity to influence the breakdown of the passivation layer that protects the underlying aluminium from corrosion. Corrosion attack preferentially takes place at weak spots or regions of loosely adherent oxides. The breakdown of the passivation layer facilitates rapid corrosion at these spots, which may result in the formation of holes (‘pits’) in the aluminium metal.

Cl^– is by far the most aggressive agent. Once Cl^– has broken down the passivation layer and has reached the metal surface, further corrosion takes place according to the reactions:

At the anode:

Al  Al³⁺ + 3e^– (12)

Followed by:

Al³⁺ + 3Cl^–  AlCl₃ (13)

And at the cathode (Cl^–-free zone):

H₂O + 1/2O₂ + 2e^– 2OH^– (14)

The resultant of these reactions is:

AlCl₃ + 3OH^–  Al(OH)₃ + 3Cl^– (15)

Reaction (15) makes it clear that Cl^– is not consumed during corrosion; rather, it remains available for further corrosion. It is therefore of utmost importance to keep the Cl^– concentration of the cooling water as low as possible.

Sufficient flow of the cooling water has also a positive effect. The generation of Al³⁺ ions in the pit attracts Cl^–, and by keeping the water in motion, the buildup of local concentrations of Cl^– and other species is avoided.

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2.8. ORGANIC COMPOUNDS

The contribution of organic compounds to corrosion effects is in most cases of minor importance; only the presence of strong organic acids and complexing agents should be avoided. Possible sources of organic compounds are: detergents, organic material leached from seals, traces organics from ion exchangers, organic traces in dust and dissolved volatile organic compounds (small aliphatic acids, alcohols, at trace level present in outdoor air).

Good practice is to keep the dissolved organic compound (DOC) concentration at subppm levels. Usually, an active carbon filter is present in the water cleaning system, a very effective tool for keeping the amount of organics in water at a low level. The measurement of DOC gives information on the amount of organic materials present in the water. Section 6 provides the recommended limits for DOC concentrations for the various reactor systems.

TABLE 2. TYPICAL GALVANIC SERIES FOR METAL/ALLOYS IN SEAWATER [10]

Platinum (most cathodic, noble, or resistant to corrosion) Gold

Graphite Titanium Silver Hastelloy C 18-8 stainless steel (passive) Chromium steel >11% Cr (passive)

Inconel (passive) Nickel (passive)

Monel Bronzes

Copper Brasses Hastelloy B Inconel (active)

Nickel (active) Tin Lead

18-8 stainless steel (active) Ni-resist

Chromium steel >11% Cr (active) Cast iron

Steel or iron 2024 aluminium

Cadmium

Commercially pure aluminium Zinc

Magnesium and its alloys (most anodic or easy to corrode)

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2.9. ALGAE

The control of algae formation in water systems is by avoidance of light, circulation of water, and avoidance of nutrients in the water.

3. DEGRADATION OF MATERIALS IN WATER

3.1. INTRODUCTION

The degradation of materials immersed in water is a very complex process. Depending on the material and the water composition, several chemical reactions can occur to form soluble or insoluble compounds. In case of metals, they can dissolve electrochemically (corrode) or can accumulate hydrogen as a result of high hydrogen fugacity at the material surface, and thereby become embrittled. The modes of attack can be thinning, pitting, and cracking by stress corrosion mechanisms (metals). Hydrogen embrittlement can also lead to cracking of metals.

Since water is a strong polar solvent, metallic materials tend to dissolve (corrode) in water and water solutions through mechanisms involving electrochemical reactions. The reactions typically involve other commonly dissolved species, namely: hydrogen ions (H⁺= protons), which comes from water dissociation; oxygen, which originates from the air and dissolves in the water as O₂; and dissolved impurity ions.

Various metallic materials are commonly used in experimental or research reactors. Particularly, in the primary circuit, stainless steels are used for structural components and fuel cladding and aluminium alloys are also used for various types of fuel cladding and structural components. Zirconium alloys are also used for fuel cladding and structural materials, and nickel-based alloys are used for heat exchangers and other minor applications. In the secondary circuit, carbon steel, and copper-based alloys are typically used. Concrete is also an important structural material that may be in contact with water storage basins. Due to the various working and environmental conditions that they are subjected to, different degradation mechanisms can occur, some of an electrochemical nature and some not (e.g. leaching), which have to be individually considered in water systems [11].

Table 3 provides a list of the important mechanisms that can degrade reactor materials in water systems.

TABLE 3. MECHANISMS OF DEGRADATION FOR REACTOR MATERIALS IN WATER General corrosion

Pitting corrosion Crevice corrosion Galvanic corrosion Intergranular corrosion Stress corrosion cracking

End-grain attack Erosion-corrosion

Blister formation Microbial corrosion Sediment induced corrosion

Leaching of calcium and silicon from concrete Carbonation and rebar corrosion in concrete

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Other degradation mechanisms of reactor components that are not related to water quality conditions (e.g.

irradiation induced loss of compressive strength of concrete) are not considered in this section.

This section describes the modes and mechanisms of attack on research reactor fuel and structural materials in reactor water systems. The basic concepts of water chemistry and its affect on materials, which were introduced in Section 2, are extended in detail for specific material systems to identify specific water quality conditions that can cause degradation. The first system considered is aluminium in Section 3.2.

3.2. ALUMINIUM AND ITS ALLOYS

3.2.1. Overview of aluminium corrosion and corresponding oxyhydroxide film formation

Aluminium is one of the most thermodynamically reactive metals, which in normal room conditions rapidly forms a thin and dense stable oxide layer (Al₂O₃) on the surface. On fresh metal, this dense oxide layer measures about 2.5 nm, and increases gradually with time. This protective oxide barrier film is strongly bonded to the surface, relatively inert, and tends to prevent further oxidation, making aluminium a metal durable for many applications, especially when exposed to air. However, when placed in a wet environment, aluminium may be strongly affected by corrosion. Aluminium oxyhydroxide (also known as aluminium hydroxide, or simply, aluminium oxide) films form and grow in wet environments. From an engineering standpoint, the kinetics or rate of corrosion of a metal is usually of primary importance. Corroding metals are not in equilibrium and therefore thermodynamic calculations cannot be applied. For metal corrosion to occur, an oxidation reaction (generally metal dissolution or oxide formation) and a cathodic reduction reaction (such as oxygen reduction) must proceed simultaneously. In most normal water environments, the overall reaction for aluminium corrosion is a reaction with water to form aluminium hydroxide and hydrogen. Within a certain pH range, the aluminium hydroxide has very low solubility in water and precipitates as bayerite or boehmite, depending on the temperature of the water [11, 12].

At low temperatures (<~80°C), the following is the predominant corrosion reaction:

2Al + 6H₂O → 2Al(OH)₃ + 3H₂ (16)

The oxidation (anodic) reaction is given by:

Al → Al³⁺ + 3e^– (17)

The reduction (cathodic) reaction is given by:

2H⁺ + 2e^– → H₂ (18)

In the electrochemical reaction, the positively charged ions leave the surface of the anode and enter the electrolyte solution, leaving electrons behind to flow through the metal to the cathode. At the cathode, the electrons are consumed by the hydrogen ions at the surface, and hydrogen gas is liberated. The oxidation and deterioration of the anode surface causes corrosion (i.e. loss of the aluminium metal).

As stated, aluminium oxyhydroxides may form and adhere to the aluminium surface; the specific type of oxyhydroxide formed depends on the temperature and pH conditions of the water [12–15]. Furthermore, aluminium is amphoteric; its oxyhydroxides are not stable and will dissolve more readily at pH levels below about 4 and above about 10 at 25°C, when compared to pH levels within that range.

The protective film (i.e. the film formed with pH in the range of 4 to 10) can also be attacked in the presence of some chemicals, which this can lead to dissolution of the metal. When the film is mechanically damaged or chemically weakened, localized corrosion in the form of pitting attack can occur because normal ‘self-healing’ or re-formation of the oxyhydroxide does not occur due to local aggressive water chemistry [16]. Additional discussion of general and pitting corrosion can be found in Ref. [17, 18] and in Section 3.2.3.

As with any diffusion controlled process, as the oxide film increases in thickness, the growth rate becomes slower with the growth kinetics tending to be parabolic. Investigations have shown that the formation of a protective oxide film on the aluminium surface at moderate temperatures occurs in three distinct stages [19], which

Good Practices for Water Quality Management in Research Reactors and Spent Fuel Storage Facilities | IAEA

Basic Principles

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IAEA Nuclear Energy Series

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Good Practices for Water Quality

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GOOD PRACTICES FOR

WATER QUALITY MANAGEMENT IN RESEARCH REACTORS

AND SPENT FUEL STORAGE FACILITIES

GOOD PRACTICES FOR

WATER QUALITY MANAGEMENT IN RESEARCH REACTORS

AND SPENT FUEL STORAGE FACILITIES

IAEA NUCLEAR ENERGY SERIES No. NP-T-5.2

COPYRIGHT NOTICE

FOREWORD

EDITORIAL NOTE

CONTENTS

1. INTRODUCTION

2. WATER CHEMISTRY

3. DEGRADATION OF MATERIALS IN WATER